1. Field of the Invention
The present invention relates generally to antennas, and more particularly to compact, broadband antennas utilizing a combination of folding and top-loading techniques.
2. Description of the Related Art
Frequency-independent antenna designs, in particular log-periodic dipole arrays (LPDAs), are widely used for broadband electric field generation applications. However, at the lower end of their operating range (the frequency range over which they exhibit frequency-independent behavior), such antennas must be approximately one-half wavelength in width. Thus, an LPDA with a lower operating frequency of 30 MHz (10 meter wavelength) must be approximately 5 meters wide. Because such dimensions are unacceptably large and because operating frequency ranges extending from below 20 MHz to above 2 GHz are required by the EMC testing industry, design techniques for a reduced-size hybrid antenna have been sought.
Reducing the size of an antenna such that its dimensions are smaller than one-half of a wave-length at its operating frequency may be described as making the antenna "electrically small". Electrically small antennas are typically defined as those which fit within a sphere having a radius of 1/2.pi. wave-lengths. Electrically small antennas are inherently more narrowband and inefficient than larger antennas, making design of compact antennas at relatively low frequencies challenging.
One common technique for extending the frequency range of an LPDA while limiting its size is the use of a broadband dipole to replace the lowest frequency element in the LPDA. For example, Brown-Woodward or bowtie dipoles can be used in conjunction with a 150 MHz LPDA (one having a low-frequency operating limit of 150 MHz) to extend the response of the antenna system down to 30 MHz. Examples of such a design are the model 3142 and 3143 antennas available from EMC Test Systems, L.P. Another possibility which is currently commercially available is to use a biconical dipole element to replace the lowest frequency elements.
While currently-available hybrid antennas are superior to LPDA antennas alone, they are still quite ineffective at the low-frequency end of their operating range. This weak performance at the low-frequency end imposes the amplifier requirements for an electric field generation system. Because the amplifier is generally the most costly component of the system, relaxing amplifier requirements would have a very significant effect on system cost.
Using dimensions and requirements typical for the EMC testing industry, we consider an antenna which occupies a 0.5 meter radius spherical volume, for which an electric field intensity of 20 V/m at 26 MHz is desired at a test distance of 3 meters from the antenna. It can be shown using radiation and power considerations that the theoretical lowest input power to achieve this performance is about 109 Watts. (It is assumed that the antenna exhibits a dipole radiation pattern.) Currently available EMC testing antennas, however, require amplifiers of at least 500 Watts to achieve this performance.
It would therefore be desirable to develop more efficient broadband antennas for operating frequencies at which the antennas are electrically small.